This invention relates to a process for the production of hydroxy-group-containing oligoesters during which no halogen is present. These oligoesters have a particularly favorable molecular weight distribution and are useful in the production of flame resistant polyisocyanate addition products.
In the field of rigid polyurethane foams, improved flame-resistance has acquired increasing significance, particularly where the foams are used in the construction industry. In view of more stringent legal requirements concerning burning behavior, it has also been necessary in recent years to intensify the search for starting materials from which it is possible to produce rigid polyurethane foams which are capable of complying with the stricter legal standards. In most attempts to satisfy these requirements, the proportion of flameproofing agent used in polyether-based foam formulations has had to be greatly increased. However, exclusive use of inexpensive, non-functional flameproofing agents, such as tris-.beta.-chloroethylphosphate, tris-.beta.-chloroisopropylphosphate or diphenylcresylphosphate, requires such large amounts of these materials that the foams obtained are unusable. It was therefore necessary to add halogenated and/or phosphorus-containing polyols to such formulations. Unfortunately such polyols add greatly to the cost of the formulations.
However, if the long-established polyether polyols, for example sugar-, glycerol- or sorbitol-started polyalkylene oxides having OH numbers of from 300 to 550, are replaced by polyester-based polyols having OH numbers in the same range, much smaller quantities of additional flame-retarding agents can be used to satisfy the relevant flame-proofing standards. There would therefore, be no need to use the expensive functional flame-retarding agents because the inexpensive, non-functional types would be sufficient.
Polyesters having OH numbers of from 300 to 550 produced by condensation of dicarboxylic acids with diols have a relatively high viscosity as a result of the natural broad oligomer distribution (Flory distribution) which makes them difficult to process. In addition, the content of unesterified free diol is extremely high. For example, a polyester of phthalic acid anhydride and diethylene glycol having an OH number of 300 contains approx. 13 wt. % free diethylene glycol. This leads to processing difficulties due to both viscosity and incompatibilities with the polyisocyanates and fluorinated hydrocarbon blowing agents used.
The above-mentioned disadvantages of viscosity and incompatibility occur to a greater extent in cases where, in addition to the diols, alcohols of relatively high functionality are used to produce the polyesters. Functionalities above 2 are desirable and sometimes necessary to obtain good foaming properties (hardening, dimensional stability, etc.). For these reasons, there has been no shortage of attempts to produce polyesters from dicarboxylic acids or dicarboxylic acid semiesters by alkoxylation reactions, i.e. reaction with alkylene oxides. Such an alkoxylation process would be advantageous because there would be a greater chance of obtaining products of narrow molecular weight distribution having a lower viscosity than corresponding polycondensates and containing very little, if any, free alcohol component. Economic considerations also support this approach because commercially inexpensive materials would be used.
The processes described in the literature do not however meet the above-mentioned expectations because either esterifications or transesterifications occur under the conditions applied, so that products of broad molecular weight distribution are ultimately formed. Alkoxylation of the OH groups present also occurs to a large degree in addition to the desired esterification. Consequently, polyether esters having a distinctly reduced flame retarding effect in rigid polyurethane foams are formed as the end products. In most cases, both the esterification and alkoxylation secondary reactions occur at one and the same time. In some cases, a very large excess of alkylene oxides has to be used to achieve a high conversion of the carboxyl groups and has to be removed by distillation on completion of the reaction. However, a procedure such as this is attended by serious disadvantages and dangers when used for production on an industrial scale.
According to U.S. Pat. Nos. 3,455,886 and 3,459,733, from 2 to 8 moles alkylene oxide per mole dicarboxylic acid semiester are required in the absence of catalysts to achieve acid numbers below 1 mg KOH/g. It was soon recognized that special catalysis is necessary for a more selective reaction. According to U.S. Pat. No. 2,779,783 or DE-A 1,568,883, alkoxylation in the presence of alkali metal halides, carbonates or hydroxides results in more selective reaction. However, alkali metal halides lead to corrosion problems and, in addition, have to be filtered off. Alkali metal carbonates or hydroxides remain as carboxylates in the polyester, unless they are neutralized and filtered, and affect the reactivity of corresponding polyurethane formulations. In addition, our own tests have shown that, under the necessary reaction conditions, transesterifications take place to a large degree, leading to increased viscosities and end products which are scarcely different from the polyesters produced by condensation (cf. Comparison Example 2a) in accordance with the prior art. In addition to transesterifications, alkali metal carbonates or hydroxides lead to a large degree to undesirable ether formation through alkoxylation of the OH groups. Large excesses of alkylene oxide are therefore necessary to obtain low acid numbers.
DE-A No. 1,248,660 and DE-A No. 3,201,203 describe the reaction of dicarboxylic acids or semiesters thereof with alkylene oxides in the presence of thiodialkylene glycols, such as thiodiglycol or thiodipropylene glycol. Although the reaction is largely unaccompanied by secondary reactions, the use of these catalysts leads to serious odor problems both during the production and during the further processing of these products. Such odors are not tolerated by the foam manufacturers. The production of polyesters by reaction of carboxylic acid anhydrides with alkylene oxides in the absence of water and in the presence of glycols and catalysts is described in U.S. Pat. No. 3,374,208. The catalysts disclosed are metal compounds with a zinc, tin, manganese, lead, nickel, cobalt or cadmium cation and an oxygen, chlorine, acetate, butyrate, phosphate, nitrate, stearate, oleate or naphthenate anion.
It is also known that carboxylic acids can be esterified with alkylene oxides in the presence of catalysts such as sulfuric acid, sodium acetate, iron(III) chloride, etc. (see Methoden der Organischen Chemie, Vol. VIII, Houben-Weyl, Georg Thieme Verlag, Stuttgart, 1952, pages 531-533). Chromium(III) compounds (e.g. chromium octoate) are used for the alkoxylation of aromatic carboxylic acids in the processes disclosed in NL-A No. 67-01261 and in BE-B No. 715 201. However, use of the chromium and other salts results in discoloration of the product which is extremely difficult to remove and requires undesirably high quantities of ethylene oxide to obtain low acid numbers. Many disclosures (FR-A No. 1,428,204, GB-B No. 623,669, GB-B No. 1,060,750, U.S. Pat. No. 2,932,622, U.S. Pat. No. 2,863,855, U.S. Pat. No. 3,414,608, DE-A No. 3,315,381) describe the use of tertiary amines such as trialkylamines, pyridine, imidazole, N-methylimidazole, phosphines or triethanolamine as catalysts. Unfortunately these amines all give rise to one or more of the disadvantages mentioned above. In addition, some amines can cause serious discoloration of the products, presumably due to quanternization of the tertiary nitrogen, so that unacceptable products are obtained. Aromatic nitrogen compounds, such as N-methylimidazole (see Comparison Example 2b infra) are outstanding transesterification catalysts and, accordingly, give products having an undesirably broad molecular weight distribution.